1. Field of the Invention
The present invention relates to a fluid-filled elastic center bearing mount for flexibly supporting a propeller shaft of a motor vehicle on the vehicle body.
2. Discussion of the Prior Art
As an intermediate bearing for supporting the propeller shaft of a motor vehicle at its longitudinally middle portion on the body of the vehicle, there has been used a so-called center bearing mount. Examples of such a center bearing mount are disclosed in laid-open Publication No. 54-29402 of examined Japanese Utility Model Application, and laid-open Publication No. 59-54316 of unexamined Japanese Utility Model Application. The center bearing mounts disclosed in these publications have a metallic inner sleeve and a metallic outer sleeve which are disposed with a suitable radial spacing therebetween, and an elastic body made of a rubber material interposed between the inner and outer sleeves. The propeller shaft is inserted through the inner sleeve of the mount via a suitable center bearing such as a ball bearing, while the outer sleeve of the mount is secured to the vehicle body, whereby the propeller shaft is flexibly connected to the vehicle body by the center bearing mount.
The center bearing mount is required to attain two different functions, namely, a function of supporting the propeller shaft, and a function of isolating or damping the vibrations of the propeller shaft to be transmitted to the vehicle body. The second function consists principally of two vibration damping or isolating capabilities or characteristics, that is, sufficient capability of damping high-amplitude vibrations having a relatively low frequency of about 5-30 Hz which occur due to secondary couples caused by a joint angle of the propeller shaft upon starting of the vehicle, and a characteristic of exhibiting sufficiently low dynamic spring constant with respect to low-amplitude vibrations having a relatively high frequency of about 50-200 Hz which occur due to a secondary component of a rotary motion of the vehicle engine transmitted to the propeller shaft.
In view of the above requirements, the assignee of the present application developed a fluid-filled elastic center bearing mount in laid-open Publication No. 61-166250 of unexamined Utility Model Application wherein an elastic body partially defining a pair of fluid chambers between the inner and outer sleeves, such that the two fluid chambers are located opposite to each other in a diametric direction of the sleeves in which input vibrations are primarily received by the mount. The fluid chambers are filled with a non-compressible fluid, and are held in communication with each other through a suitable orifice. In this type of fluid-filled elastic center bearing mount, the input vibrations applied to the inner and outer sleeves are damped or isolated based on resonance of the fluid which flows through the orifice due to pressure variations in the fluid chambers caused by a relative displacement of the two sleeves.
However, the range of the frequency of the vibrations that can be effectively damped or isolated based on the fluid mass resonance in the center bearing mount constructed as described above is limited or determined by the specific configuration and dimensions of the orifice. If the orifice is tuned so as to effectively damp low-frequency vibrations, the orifice tends to be placed in a condition as if the orifice was closed, when the input vibrations have a frequency higher than that of the low-frequency vibrations to which the orifice is tuned. Consequently, the center bearing mount exhibits an excessively high dynamic spring constant upon application of the high-frequency vibrations, and suffers from comparatively low vibration isolating capability for the high-frequency vibrations.
Thus, the fluid-filled elastic center bearing mount described above is still unsatisfactory or has some room for improvements, particularly in terms of the frequency range of the vibrations that can be suitably and effectively damped or isolated, for assuring enhanced driving comfort of the vehicle.